Integrated device for controlling charging artifacts in scanning electron microscopes

ABSTRACT

A device is disclosed which permits neutralization of specimen charging in electron microscopes so that charging interference is eliminated from output image information. The device also enables contrast adjustments to be made in the specimen zone rather than in the output signal processing apparatus. The device includes an apparatus for generating a vapor in the specimen region and further includes a vapor deflection shield structure which carries an adjustable bias voltage for permitting contrast control. The method of operating the device of the present invention in conjunction with a scanning electron microscope is also disclosed.

United States Patent Cohen et al.

1451 Nov. 11, 1975 0 DC SUPPLY [5 INTEGRATED DEVICE FOR 857.253 11/1952Germany 250/511 CONTROLLING CHARGING ARTIFACTS IN SCANNING ELECTRONMICROSCOPES Primary E.\'uminer.lames W. Lawrence [75] Inventors: ArthurL. Cohen, Pullman, Wash; 4535mm EMWII'IIW-T Grigsby Gerald Gamer, SantaMonica, Armrney, Age/1r. or F1rmOblon. Fisher. Spn'ak. Califi; RaymondG. E. Steever. Jr., Mccleuand & Miller Pullman. Wash.

[73] Assignee: Research Corporation, New York, [57] ABSTRACT NY. {22]Filed: Apr 12, 1974 A devlce is disclosed which perm1ts neutrahzation ofspeclmen charging 1n electron microscopes so that [21] Appl. No.:460,555 charging interference is eliminated from output imageinformation. The device also enables contrast adjust- [52 US. (:1.250/311; 250/306 m f the rather 9 2 the output signal processingapparatus. The deuce in- [51] Int. Cl. GOlN 23/00 eludes ,m uppamtus foroenemting a vapor in the Spec [58] Field of Search 250/306. 3ll. 310.307 I I H lmen region and further Includes a \apor deflectlon shieldstructure which carries an adjustable bias volt- [56] References Citedage for perm1ttmg contrast control. The method of op UNITED STATESPATENTS crating the device of the present invention in conjunc-Z.890.342 6/1959 Columbe .1 250/306 tion with a scanning electronmicroscope is also dis 3.505.52l 4/1970 Wegmann et al 250/3 ll l dFOREIGN PATENTS OR APPLICATIONS 0 Cl 6 D F 4624460 7/1971 Japan 250/310rawmg gums DICATOR REVERSIBLE POLARITY DC SUPPLY US. Patent Nov. 11,1975 Sheet10f3 3,919,553

US. Patent Nov. 11,1975 Sheet 2 of3 3,919,553

01o i900V 22 H6. 4 DC SUPPLY II,

| 80 76 H l6 5) v 7 L) h 50 86 VARIABLE 0c TEMPERATURE VOLTAGE SUPPLY IINDICATOR 71 7 1 as H8 32/ 6o 70 32 f 8" 82 3 1 28 as 36 as POLARITY 0cSUPPLY w TL 1 SEN BEAM GENERATING AND FOCUSING SYSTEM US. Patent Nv.11,1975 Sheet3of3 3,919,553

l \L[ 50 I00 200 300 u 900 DEFLECTION SHIELD VOLTAGE mmammmmm momsTEMPERATURE (C) INTEGRATED DEVICE FOR CONTROLLING CHARGING ARTIFACTS INSCANNING ELECTRON MICROSCOPES BACKGROUND OF THE INVENTION 1. Field ofthe Invention:

- This invention relates generally to electron microscope controlapparatuses and more particularly to a device for controlling charginginterference and image contrast in scanning electron microscopes, and tothe defined as those parts of an output image signal which are out ofaccord with the expected intensity or spatial and temporal distributionof the electrons received by an electron microscope collector. Thesignificance of charging artifacts, which may also be described ascharging interference or image interference due to specimen charging,may be readily understood by reference to FIGS. 1 and 2 which are bothelectron photomicrographs of a single specimen of untreated moth tongue,both showing the specimen magnified 600 times.

I The photomicrograph of FIG. 1 demonstrates the occurrence of chargingartifacts. In particular, a large portion of the moth tongue is shown asbeing greatly over-exposed to the extent that no image contrast existsalong the central portion of the specimen, so that all detail andconsequently, all useful image information is lost in this area. Thephotomicrograph of FIG. 2 shows the same specimen as in FIG. I, and wasmade in the same manner as the photomicrograph of FIG. I, with theexception of the fact that the present invention was used in making thephotomicrograph of FIG. 2 to prevent the development of chargingartifacts and to properly adjust the image contrast. As is apparent frominspection of these figures, the specimen image quality is greatlyimproved in FIG. 2.

The charging artifacts illustrated in FIG. 1 are caused by theaccumulation of an electrical charge on the specimen surface due to theimpingement of an electron beam on the specimen as it is examined undera scanning electron microscope (SEM). The conductivity of the specimen,the existence of neighboring charged surfaces, the energy of theimpinging electron beam and the exposure parameters of the specimen tothe electron beam are significant factors in determining the extent towhich charging artifacts are likely to'occur.

Devices for reliably controlling charging artifacts and for controllingSEM image contrast in the region of the specimen are believed to beessentially unknown in the prior art. However, at least one devicehaving some similarity to the apparatus of the present invention isknown. This device is called an environmental control stage and wasdescribed in a paper by W. C. Lane included in the Proceedings of theThird Annual Scanning Electron Microscope Symposium, IIT ResearchInstitute, Chicago, Illinois. The device described in this publicationis designed to form a water vapor zone in the area of a wet specimen,that is, a specimen whose properties are best maintained'in a wet ormoist environment. The Lane apparatus includes a reservoir and a heaterapparatus for heating a liquid or solid and producing a vapor which isdirected through small channels directly at a specimen mounted on astub.

The apparatus disclosed in the Lane paper does not, however, discloseany type of device for controlling image contrast, nor does it suggestthe use of vapor in the region of a specimen to control chargingartifacts. Thus, while the device disclosed in the Lane paper includessome similarities to the structure of the apparatus of the presentinvention, the Lane apparatus does not suggest the purpose or mode ofoperation of the present invention, and does not suggest or imply thatimage improvement or image control in electron microscopes can beachieved using the device disclosed therein. Accordingly, the presentinvention is believed to provide a substantial step forward in the artof SEM image control and image enhancement.

SUMMARY OF THE INVENTION Accordingly, one object of this invention is toprovide a novel apparatus for controlling charging artifacts in electronmicroscopes.

Yet another object of this invention is the provision of a novelapparatus for controlling image contrast in electron microscopes.

A still further object of the present invention is the provision of anovel method for reducing charging interference in electron microscopes.

A still further object of the present invention is the provision of anovel method of controlling image contrast in electronmicroscopes bycontrolling the potential of an apparatus in the specimen zone.

Yet another object of the present invention is the provision of a novelapparatus for producing a vapor in the region of a specimen mounted inan evacuated electron microscope housing.

Another object of the present invention is the provision of a novelmethod for controlling charging artifacts by generating a vapor in theregion of a specimen in an evacuated environment.

A still further object of the present invention is the provision of anovel method for controlling image contrast in electron microscopes byvarying the potential of a conductive shield structure positioned in thevicin ity of a specimen.

Briefly, these and other objects of the present invention are achievedby providing a stub holder assembly including a vaporization chamberhaving a heater attached thereto, and having a deflection shieldpositioned around the sample stub. A suitable chemical is placed in thevaporization chamber and is vaporized by the application of heat to thevaporization chamber. The resulting vapors are channeled via thedeflection shield to the vicinity surrounding a sample mounted on thestub for controlling charging artifacts. A potential source is coupledto the deflection shield for adjusting the bias voltage thereof wherebyimage contrast may be varied.

BRIEF DESCRIPTION OF THE DRAWINGS A more complete appreciation of theinvention and many of the attendant advantages thereof will be readilyobtained as the same becomes better understood by reference to thefollowing detailed description when considered in connection with theaccompanying drawings wherein:

FIG. I is a photomicrograph taken at a 600 power magnification showingcharging artifacts on a sample consisting of an untreated moth tongue;

FIG. 2 is a photomicrograph identical to that of FIG. 1 but showing theimproved contrast and absence of charging artifacts when the presentinvention is used to control the image quality;

FIG. 3 is a schematic and block diagram showing the relationship of thepresent invention to a scanning electron beam microscope system;

FIG. 4 is a cut-away side view of the apparatus of the present inventionshowing in detail the components thereof;

FIG. 5 is a graphical representation of output signal level as afunction of deflection shield voltage using the apparatus of the presentinvention; and,

FIG. 6 is a graphical representation of output signal level as afunction of vaporization chamber temperature using the apparatus of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings,wherein like reference numerals designate identical or correspondingparts throughout the several views, and more particularly to FIG. 3thereof, the relationship of the apparatus of the present invention to aconventional scanning electron microscope system is illustratedschematically. The device 10 of the present invention, which may bereferred to as an integrated device for controlling charging artifactsand image contrast in an SEM, is shown mounted to a stage 12 of aconventional scanning electron microscope. A sample 14 is mounted to aconventional stub 16 held by the device 10. The stub 16 is oriented atan angle of approximately 45, for example, to an impinging electron beam18 emanating from a conventional SEM beam generating and focusing system20. The electron beam 18 passes through a cut-away portion of adeflection shield 22 to scan the sample 14. Electrons emitted from thesample 14 are directed toward a conventional SEM collector 24, theoutput of which is applied to a conventional SEM imaging system 26.

Having thus established the relationship of the device 10 of the presentinvention to a conventional SEM system, attention is now directed toFIG. 4 wherein the device 10 of the present invention is illustrated ingreater detail. The device 10 includes a central body portion 28,preferably formed of a material which has good heat conductingproperties, such as brass, copper or the like. The central body portion28 partially encloses a vaporization chamber 30 wherein a suitablematerial, such as chloral hydrate is placed when the device 10 is beingoperated. A plurality of apertures 32, shown by dashed lines, aredrilled around the periphery of central body portion 28 to provide clearexhaust channels for vapors generated within the vaporization chamber30.

The central body portion 28 includes a lower appendage 34 which fitsinto a suitably sized aperture in a heat sink 36. The heat sink 36 ispreferably formed of copper or a similar material having very good heatconducting properties. The lower appendage 34 may be press-fitted,threaded or otherwise affixed to the heat sink 36 as long as theappendage 34 and the heat sink are in close engagement to permit anunimpeded flow of heat energy therebetween.

The heat sink 36 is secured by means of plurality of bolts 38, or othersuitable fasteners, to a pair of heat transfer and mounting plates 40. Astage mounting screw 42 is coupled to the heat transfer and mountingplates 40 to permit the device 10 to be secured to the stage 12 of aSEM, as shown in FIG. 3.

A thermoelectric device 44 is secured between the heat sink 36 and theheat transfer and mounting plates 40 by the tension applied to bolts 38.The thermoelecr. tric device 44 is preferably a conventional,commerically available Peltier heater, and is coupled to a suit, ablereversible polarity DC supply. As will be apparent to those skilled inthe art, the Peltier heater operates according to the well known Peltiereffect whereby heat is produced or absorbed at the junction of twometals when current is passed through the junction. More particularly,heat generated by a current flowing through the junction in onedirection will be absorbed when the current is reversed. This type ofheat source is particularly suitable in the environment of the presentinvention since the Peltier heater is arranged such that when the upperportion thereof generates heat, the lower portion is cooled. Thus heatmay be supplied to the heat sink 36 but not to the microscope stage 12through the heat transfer plates 40. This provides a substantialadvantage in allowing rapid temperature changes to be made simply byreversing the polarity of the DC supply 46. Rapid temperature changescould not be made if other types of conventional heaters, such asfilament heaters, were used, since such heaters have the effect ofsupplying the same quantity of heat to the heat transfer plates 40 andthe microscope stage 12 as is supplied to the heat sink 36. Accordingly,when such conventional non-directional heat sources are used, themicroscope stage is heated to a high degree and requires a considerabletime period to cool, preventing rapid temperature changes at thelocation of the vaporization chamber 30. The Peltier heater can also, ofcourse, be used to cool rather than heat the heat sink 36 and thevaporization chamber 30.

The upper end of central body portion 28 includes a cylindrical aperturedefined by inwardly curved side walls 48. The shape of the side wallsslightly restricts the flow of vapor from the vaporization chamber 31)and serves to partially enclose the upper portion of the vaporizationchamber.

An ionization ring 50 constructed of an insulating material such asTeflon (TM) is press-fitted to the upper portion of side walls 48 ofcentral body portion 28. The ionization ring 50 includes two fine wires51 formed of non-corrosive metal and positioned within suitable grooves52 formed around the inner periphery thereof. The wires 51 may, forexample, be formed of a gold palladium alloy, or any other suitablynon-corrosive metal or alloy. The wires 51 are coupled to a conventionalDC voltage supply capable of generating a continuously variable staticoutput potential of from O to 5 kv. The wires 51, which are preferablyplaced approximately 3 millimeters apart and located just below the topsurface of the stub 16 are intended to supply a potential for ionizingvapor molecules rising from the vaporization chamber 30, as will beexplained in greater detail subsequently. However, the apparatus of thepresent invention functions adequately without the use of the ionizationwires 51, and thus they may be omitted from the ionization ring 50, orthe device 10 may be operated with the variable voltage supply 54switched off. When the ionization wires 51 are omitted, or when no poweris supplied to them, the ionization ring 50 functions solely as aninsulator to insulate the deflection shield 22 from the body of thedevice, allowing it to maintain any desired applied charge, as will beapparent to those skilled in the art.

The previously mentioned deflection shield 22 is press-fitted to anupper surface of the ionization ring 50. The deflection shield 22 ispreferably constructed of a highly conductive material having an innerdiameter of approximately 1.3 centimeters and having a partiallycut-away side wall structure, as shown in FIG. 3, to permit the SEM beam18 to reach the sample 14 placed on the surface of stub 16. Thedeflection shield 22 is externally connected to a continuously variable,preferably well filtered DC power supply 56 which is preferably capableof producing a continuously variable output potential of from O to 900volts. The shape of the deflection shield prevents line-of-sight pathsbetween the sample 14 and the collector 24, thus eliminating thepossibility of high voltage discharge light which might occur in theionization system of the present invention. The main purpose of thedeflection shield, however, is to channel vapors rising from thevaporization chamber 30 into the area occupied by the sample 14, as willbe explained in greater detail subsequently.

A stub holder 58 is positioned within the opening defined by thedeflection shield 22, the ionization ring 50 and the vaporizationchamber 30. The stub holder 58 includes a body 60 having a threadedmounting and adjusting screw 62 extending downwardly therefrom forcooperating with a threaded aperture in the central body portion 28 ofdevice 10. A coil spring 64 is positioned around the mounting andadjusting screw 62 for preventing inadvertent rotation of the stubholder 58.

The body 60 of the stub holder 58 includes an axial aperture 66 foraccomodating a mounting leg 68 of stub 16. A detent ball 71 is movablymounted in a side aperture 70 of the body 60, and is biased intoengagement with the mounting legs 68 by means of a resilient ring 74,which may be a conventional elastic O-ring, for example.

A vapor flow control flange 76 is formed integral with the upper portionof body 60 of the stub holder 58. The vapor flow control flange 76includes an inner surface which is sloped away from the stub 16 and alsoincludes an outer surface 78 which is angled toward the wall ofdeflection shield 22.

As will be apparent from FIG. 4, the stub holder 58 is designed tosecure one Cambridge pin type stub, for example recessed into theconical, flanged depression defined by the vapor flow control flange 76.A small vapor gap 80 is formed between the outer surface 78 of the vaporflow control flange 76 and the inner periphery of the ionization ring50. The vapor gap 80 can be varied from a very thin circular slit to asignificant gap depending upon the extent to which the mounting andadjusting screw 62 is threaded into its cooperating aperture. The stubholder is also preferably electrically grounded, as indicated at 82. Athermistor probe 84 is preferably mounted in the side wall 48 of thecentral body portion 28 to monitor the temperature of the vaporizationchamber 30. The output of the thermistor probe 84 is coupled to aconventional temperature indicator 86 to provide a convenient readout ofthe vaporization chamber temperature.

Having described in detail the structure of the present invention, themethod of operation thereof will be more fully explained. A suitablechemical compound was first selected for generation of the desiredvapor. Chloral hydrate was selected as the preferred compound since itis normally a solid, and thus convenient to handle, and possesses avolatility such that at ambient temperature its vapor pressure is lowenough to permit satisfactory pump down of the SEM vacuum system.However, once the temperature is raised sufficiently, this compoundvolatilizes sufficiently for its gaseous ions to conduct away charges.It is noted that the decomposition products of chloral hydrate may behighly toxic, and thus a foreline trap is advisable in the SEM vacuumsystem.

To reduce the initial volatization of the chloral hydrate, three or fourcrystals of the material are first preferably wrapped in Teflon tape inwhich several puncture holes are subsequen'tially made to permit vaporsto escape. This arrangement permits a suitable vacuum to be achieved inthe SEM system at a more rapid rate, although it is not crucial to theoperation of the present invention.

The crystals thus wrapped are placed into the vaporization chamber 30and the stub holder 58 is subsequentially screwed into place above thecrystals until the vapor gap is approximately 0.5 millimeters wide. Thestub 16 with the specimen 14 attached thereto is then inserted into thestub holder 58, and the deflection shield 22 is then snapped on. It isassumed at this point that the device 10 is mounted to the SEM stage 16,and thus evacuation of the microscope chamber is begun at this time.

When the microscope chamber is fully evacuated, the reversible polarityDC supply 46 is switched on and the polarity is selected so that heat isapplied from the thermoelectric device 44 to the heat sink 36. In thismanner the vaporization chamber 30 may be heated to a suitable operatingtemperature, such as 80C. The temperature indicator 86 is used tomonitor the temperature of the vaporization chamber, as will be apparentto those skilled in the art.

Heating of the chloral hydrate crystals contained in the vaporizationchamber 30 causes the crystals to vaporize, with the result that theemitted vapors rise toward the deflection shield through the apertures32 around the peripheryof the vaporization chamber 30. It will be notedthat a near vacuum exists in the region of the device 10, and. thus thevapors rise in a virtually straight line. Accordingly the rising vapormolecules will impinge upon the angled outer surface 78 of the vaporflow controlflange and will be directed along a line parallel to theangle of the surface 78 into the side wall of deflection shield 22. Thedeflection shield will deflect these molecules back into the regionoccupied by the sample 14 so that the vapor molecules may engage thesurface of the sample 14 and neutralize any charge developing onthesurface of the sample 14.

The variable power supply 54 may also be energized to supply an ionizingpotential to the wires 51 for the purpose of ionizing vapor molecules asthey rise through the ionization ring 50. The ionized molecules have asomewhat greater ability to neutralize charges developing on the sample14. However, the apparatus of the present invention can be operatedwithout ionization of the rising vapor molecules.

The contrast of the output information appearing on the imaging system26 (FIG. 3) may be varied by adjusting the output of voltage supply 56to change the bias potential of deflection shield 22. It has been foundthat potentials in the range between 0 and 00 volts are most effectivein adjusting image contrast. In this regard attention is directed toFIG. 5 which is a graphical representation of output signal level(vertical axis) a curve 90 represents background signal level. Thedistance between curves88 and 90 represents'the image contrast in theoutput signah FIG. illustrates that the image contrast is sharplyreducedwhen negative potentials are applied to the deflection shie ld22and ismore gradually reduced when positive potentials are applied. Suchcontrol over image contrastis useful in all situations in which normalimagecontrast is too great for convenient interpretationor recording ofthe output image. In some cases, for example,-image contrast extendsbeyond the range of sensitivity of conventional photographic recordingfilms so that a moderate reduction in contrast results in improved finalimages. In other situations the contrast between two portions of aspecimen image is so great that the entire specimen image cannot beclearly observed or recorded. In such situations the contrast reducingcapability of the present invention greatly improves the clarity of theoutput image.

It is further illustrated in FIG. 5 that any biasing of the deflectionshield causes a drop in the signal level which must be compensated forin the video amplification of the microscope imaging system. However,while deflection shield biasing tends to reduce the output signal level,vapor presence appears to raise the signal level somewhat as will beapparent by reference to FIG. 6.

FIG. 6 is a graphical representation of output signal level (right handvertical axis) versus vaporization chamber temperature (horizontalaxis). The vapor pressure of chloral hydrate is also represented. Moreparticularly the vapor pressure of chloral hydrate is represented by acurve 92 in FIG. 6, while a curve 94 represents background signal level,a curve 96 represents object signal level in the presence of ionizedchloral hydrate molecules and a curve 98 represents the object signallevel in the presence of un-ionized chloral hydrate molecules. FIGS. 5and 6 show data conditions for a non-charging sample.

It will be seen from a comparison of FIG. 5 and 6 that the presenceofvapor tends to enhance the signal level i to at least partiallycompensate the decrease in the signal level due to the deflection shieldbiasing.

Although chloral hydrate was previously mentioned as the'preferredcompound for use with the present invention, it should be noted thatother compounds such as Freon TF (TM), paradichlorobenzene and otherorganic halogen compounds may be also conveniently used.

It should also be noted that while the preferred embodiment of theinvention includes a chamber within the device 10 for generating thedescribed charge-neutralizing vapor, the vapor may be introduced intothe chamber 30 from a reservoir external to the main body of the device.For example, the vapor may be purchased from a commercial producer andintroduced into the chamber 30 from a suitable tank.

It is believed clear that the teachings set forth herein identify tothose skilled in the art a method and apparatus for both controllingcharging artifacts and controlling image contrast in ,the specimen zonewhich will substantially enhance and improve theperformance ofconventional scanning electron microscopes,

Obviously, numerous modifications and variations of the presentinvention are possible in lightof the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed as new and desired to be secured by letters patent ofthe United States is:

l. A method for controlling electron beam specimen charging in anelectron microscope comprising the steps of:

introducing a charge-neutralizing vapor into said electron microscope,directing said vapor into a region within said microscope at which saidspecimen is normally positioned; and, neutralizing charges'accumulatedon said specimen due to impingement of said electron beam on said sampleby contacting said sample with said vapor. 2. A method as in claim 1,wherein said step of introducing includes the step of:

generating said vapor within said electron microscope. 3. A method as inclaim 1, wherein said step of introducing further comprises the step of:

heating a substance which normally exists in a nonvaporous state untilvaporization occurs. 4. A method as in claim 1, wherein said step ofintroducing includes the step of:

vaporizing chloral hydrate crystals. 5. A method as in claim 4, furthercomprising the step of:

placing said chloral hydrate in an evacuated chamber within saidelectron microscope prior to said step of heating. 6. A method as inclaim 1, further comprising the step of:

ionizing said vapor prior to said step of neutralizing. 7. A method asin claim 6, wherein said step of ionizing includes the step of:

passing said vapor by a pair of charged conductors. 8. An apparatus forcontrolling charging artifacts and for controlling image contrast inelectron microscopes comprising:

body means in said apparatus for defining a vaporization chamber, heatcontrol means thermally coupled to said body means for controlling thetemperature of said vaporization chamber, specimen holding means coupledto said body means for supporting a specimen in said apparatus; and,electrode means secured to and electrically insulated from said bodymeans for supplying an image contrast control potential to a regionadjacent said specimen holding means. 9. An apparatus as in claim 8,wherein said heat control means comprises:

a thermoelectric device for heating and cooling said vaporizationchamber. 10. An apparatus as in claim 9, wherein said thermoelectricdevice comprises:

a device which produces or absorbs heat in accordance with the Peltiereffect. 11. An apparatus as in claim 9, further comprising: heat sinkmeans mounted between said thermoelectric device and said body means forstoring heat; and, temperature indicating means coupled to said bodymeans for monitoring temperatures within said vaporization chamber.

9 12. An apparatus as in claim 8, further comprising: variable directcurrent voltage supply means adapted to be coupled to said electrodemeans for applying an adjustable contrast control voltage thereto. 13.An apparatus as in claim 8, further comprising: ionization means coupledbetween said body means and said electrode means for applying anionizing potential to vapors emanating from said vaporization chamber.14. An apparatus as in claim 13, wherein said ionization meanscomprises:

a ring of insulating material having a plurality of conductors mountedto an interior surface thereof. 15. An apparatus as in claim 14, furthercomprising: variable voltage supply means coupled to said conductors forapplying an adjustable ionizing potential thereto. 16. An apparatus asin claim 8, wherein said specimen holding means comprises:

stub holder means for holding a specimen stub; and vapor flow controlflange means integral with said stub holder means. i 17. An apparatus asin claim 16, wherein said specimen holding means further comprises:

threaded means for fastening said specimen holding means to said bodymeans and for adjusting the position of said vapor flow control flangewhereby vapor flow from said vaporization chamber may be regulated.

18. An apparatus as in claim 16, wherein said stub holder meanscomprises:

a structure having an axial aperture therein for re ceiving a stubmounting leg; and

detent means resiliently mounted in said structure for engaging saidstub mounting leg.

19. Apparatus as in claim 8, further comprising:

vapor flow control means including said electrode for directing vaporgenerated in said vaporization chamber into a region adjacent saidspecimen holdingme'ans.

20. An apparatus for controlling charging artifacts and for controllingimage contrast in electron microscopes comprising:

body means in said apparatus defining a chamber,

means for supplying a charge-neutralizing vapor to said chamber,

specimen holding means coupled to said body means for supporting aspecimen in said apparatus; and, electrode means secured to said bodymeans for supplying an image contrast control potential to a regionadjacent said specimen holding means.

1. A method for controlling electron beam specimen charging in an electron microscope comprising the steps of: introducing a charge-neutralizing vapor into said electron microscope, directing said vapor into a region within said microscope at which said specimen is normally positioned; and, neutralizing charges accumulated on said specimen due to impingement of said electron beam on said sample by contacting said sample with said vapor.
 2. A method as in claim 1, wherein said step of introducing includes the step of: generating said vapor within said electron microscope.
 3. A method as in claim 1, wherein said step of introducing further comprises the step of: heating a substance which normally exists in a non-vaporous state until vaporization occurs.
 4. A method as in claim 1, wherein said step of introducing includes the step of: vaporizing chloral hydrate crystals.
 5. A method as in claim 4, further comprising the step of: placing said chloral hydrate in an evacuated chamber within said electron microscope prior to said step of heating.
 6. A method as in claim 1, further comprising the step of: ionizing said vapor prior to said step of neutralizing.
 7. A method as in claim 6, wherein said step of ionizing includes the step of: passing said vapor by a pair of charged conductors.
 8. An apparatus for controlling charging artifacts and for controlling image contrast in electron microscopes comprising: body means in said apparatus for defining a vaporization chamber, heat control means thermally coupled to said body means for controlling the temperature of said vaporization chamber, specimen holding means coupled to said body means for supporting a specimen in said apparatus; and, electrode means secured to and electrically insulated from said body means for supplying an image contrast control potential to a region adjacent said specimen holding means.
 9. An apparatus as in claim 8, wherein said heat control means comprises: a thermoelectric device for heating and cooling said vaporization chamber.
 10. An apparatus as in claim 9, wherein said thermoelectric device comprises: a device which produces or absorbs heat in accordance with the Peltier effect.
 11. An apparatus as in claim 9, further comPrising: heat sink means mounted between said thermoelectric device and said body means for storing heat; and, temperature indicating means coupled to said body means for monitoring temperatures within said vaporization chamber.
 12. An apparatus as in claim 8, further comprising: variable direct current voltage supply means adapted to be coupled to said electrode means for applying an adjustable contrast control voltage thereto.
 13. An apparatus as in claim 8, further comprising: ionization means coupled between said body means and said electrode means for applying an ionizing potential to vapors emanating from said vaporization chamber.
 14. An apparatus as in claim 13, wherein said ionization means comprises: a ring of insulating material having a plurality of conductors mounted to an interior surface thereof.
 15. An apparatus as in claim 14, further comprising: variable voltage supply means coupled to said conductors for applying an adjustable ionizing potential thereto.
 16. An apparatus as in claim 8, wherein said specimen holding means comprises: stub holder means for holding a specimen stub; and vapor flow control flange means integral with said stub holder means.
 17. An apparatus as in claim 16, wherein said specimen holding means further comprises: threaded means for fastening said specimen holding means to said body means and for adjusting the position of said vapor flow control flange whereby vapor flow from said vaporization chamber may be regulated.
 18. An apparatus as in claim 16, wherein said stub holder means comprises: a structure having an axial aperture therein for receiving a stub mounting leg; and detent means resiliently mounted in said structure for engaging said stub mounting leg.
 19. Apparatus as in claim 8, further comprising: vapor flow control means including said electrode for directing vapor generated in said vaporization chamber into a region adjacent said specimen holding means.
 20. An apparatus for controlling charging artifacts and for controlling image contrast in electron microscopes comprising: body means in said apparatus defining a chamber, means for supplying a charge-neutralizing vapor to said chamber, specimen holding means coupled to said body means for supporting a specimen in said apparatus; and, electrode means secured to said body means for supplying an image contrast control potential to a region adjacent said specimen holding means. 